Perform clean just before emiting files to output dir

[ChrisRAoW]

Currently the files are removed at the beginning of the compiliing process. For production environments that isn't really what you want, because:

• During compilation the assets aren't available
• When compilation fails, assets are done.

So basically you want the old assets to be cleaned just before the new files are emitted to the output dir.

This PR does exactly that by using the event hook of the compiler named "emit".

The only thing you have to do is setting "beforeEmit" to true in the options.

[coveralls]

Coverage decreased (-3.3%) to 87.156% when pulling 93cd388 on chrisschrijver:master into 01d8dda on johnagan:master.

[coveralls]

Coverage decreased (-3.3%) to 87.156% when pulling a2f6a12 on chrisschrijver:master into 01d8dda on johnagan:master.

[factoidforrest]

I really, really like this option. I have noticed many times that things end up in a weird state when a build fails.

Obviously, it's not the right choice some of the time, but sometimes you really need it.

[johnagan]

awesome addition. thank you!

[ChrisRAoW]

You're welcome. Happy to contribute.